1. Technical Field
Embodiments of the present invention relate to techniques of generating image data with an imaging device, such as a digital camera. More specifically embodiments of the invention pertain to techniques of reducing signal overshoots and undershoots in demosaicking a mosaic arrangement of fine color filters of R (red), G (green), and B (blue) corresponding to three primary colors of light.
2. Related Art
With the advancement of digital techniques, images are generally processed as digital data (image data). Imaging devices such as digital cameras enable immediate output of captured images in the form of image data. The imaging device is typically equipped with an electronic image sensor consisting of small elements for converting the light intensities into electric signals. The imaging device focuses a captured image of a subject on the image sensor by means of an optical system and detects the light intensities in the individual elements as electric signals to generate image data. The light entering the optical system may be divided into three color components R, G, and B corresponding to three primary colors of light. The respective color lights of the three color components R, G, and B are directed to the image sensor, and the electric signals representing the light intensities of the respective color components acquired by the sensor are output to generate color image data. It is noted that G components are often referred to as ‘luminance’ components whereas R and B components are often referred to as ‘chrominance’ components.
The simplest method of acquiring the respective color lights of the three color components R, G, and B, which are obtained as divisions of the light entering the optical system, by the image sensor uses a spectroscopic prism to divide the incident light into the color lights of the three color components R, G, and B and focuses the respective color lights on image sensors to generate image data with regard to the respective color components R, G, and B. This method undesirably requires the three image sensors. Therefore, an imaging device relying on three image sensors to capture color images is occasionally called a ‘three image sensor’ device.
To reduce the cost of an imaging device, one extensively used technique, known as a color filter array, allocates one of the R, G, and B color components to each of the light-sensitive elements constituting the image sensor to attain detection of the respective color components R, G, and B by one image sensor. A typical configuration of this technique provides small color filters allowing transmission of only the R component in front of the photo-elements assigned for detection of the R component, small color filters allowing transmission of only the G component in front of the elements assigned for detection of the G component, and small color filters allowing transmission of only the B component in front of the elements assigned for detection of the B component. Since each element assigned for detection of a predetermined color component (for example, the R component) is unable to detect the other color components (for example, the G component and the B component), the resulting image data accordingly has a mosaic arrangement of pixels of the R component, pixels of the G component, and pixels of the B component. Interpolation of missing color components in each pixel with color components of adjacent pixels enables generation of color image data with the settings of all the color components R, G, and B in all the pixels. The process of interpolating the missing color components in the image data of the mosaic arrangement to generate color image data with the settings of all the color components R, G, and B is sometimes referred to as a ‘demosaicking process’. An imaging device that uses only one image sensor covered by a color filter array is occasionally called a ‘single image sensor’ device.
Since natural color images exhibit significant spectral correlation, demosaicking processes typically use various color correlation assumptions to minimize the processing errors which usually cause color shifts and artifacts in demosaicked images. Due to its simplicity and effectiveness, a correlation model based on the assumption of uniform color differences in a localized neighborhood is a de-facto default element in most demosaicking solutions. However, the underlying modeling assumption often breaks down in chromatic image areas with high-contrast edges, thus resulting in the so-called signal undershoots and overshoots, which are demosaicked values that are smaller or larger than the values of the neighboring original color components corresponding to the same color band as the demosaicked sample under consideration. Significant signal undershoots and overshoots are perceived as color artifacts present in the demosaicked images. Color artifacts degrade the picture quality of demosaicked images.